Most of these are poorly struck but they shouldn't all be. It's easier to believe you're seeing coins with light wear though it can be very difficult to differentiate between weak strikes and wear.
Wear always starts at the high points of a coin and erodes deeper and deeper. Tip the coin sideways between your eye and a light source back and forth and the high points will seem to just rise above the surface. Wear appears as a light greyish discoloration on the high points. If only some high points are discolored like this you might suspect weak strike. Weak strikes often has more of a granular appearance while wear doesn't.
On the '70's era halfs the reverse high point is the shield. Look at the top of the shield for marking and wear. Even gems will often have light marking on the top of the shield as this is very fragile here.
Time don't fly, it bounds and leaps.